Electroluminescent device



July 14,1959 J'MWNTYRE ETA? 2,894,854

' ELECTROLUMINESCENT DEVICE Filed July 29, .1958 2 Sheets-Sheet 1 uctlve Layer rro- Electrlc Materlal P q |e lc Transparent Conductlve Film Transparent Base "fFerr'o-Electrlc Material and Phosphor Dielectrlc v Transparent Conductive Film -Transparent Base Alfred J. Maclnry re, Jr.,

R ichord A. Marrel INVENTO/RS,

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July 14, 1959 A. J. M INTYRE, JR, ETAL ELECTROLUMINESCENT DEVICE 2 Sheets-Shem; 2

Filed July 29, 1958 250 V0 LTAG E mwmzkzwim VOLTAGE O O O 9 7 5 mwmzkIwEm Alfred J.Mocln1yre, Jr., Richard A. MorTel N VE N TORS.

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ATTORNEY United States Patent (3 ELECTROLUMINESCENT DEVICE Alfred J. 'Maclntyre, Jr., Los Angeles, and Richard A. Martel, Gardena, Calif., assign'ors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application July 29, 1958, Serial No. 751,680 10 Claims. (Cl. 11733.5)

This invention relates to devices for producing the emission of light by the luminescence of a phosphor material in an alternating electrical field. More particularly, the invention relates to electroluminescent devices capable of producing more light than heretofore attainable.

By now electroluminescent devices are well-known in the art. In general, these devices comprise a pair of electrodes in the form of layers with an electroluminescent phosphor material layer sandwiched therebetween. At least one of the electrodes is light-transparent so that the light produced by luminescence of the phosphor material can be observed. The purpose of the electrodes, of course, is to establish an alternating electric field across the phosphor material which results in luminescence.

The amount of light emitted by an electroluminescent phosphor is governed by several factors. An increase in light output may be attained by increasing the voltage across the electroluminescent phosphor layer. The usual limiting factor in increasing light by this method is the dielectric strength of the phosphor layer which is relatively low. it has been proposed to incorporate between the electrodes a material of high dielectric strength in order to permit the utilization of greater voltages. To this end the electroluminescent phosphor itself has been dispersed in a material such as a transparent plastic having a much higher dielectric constant. Likewise layers of such a dielectric plastic material have been disposed between the electrodes, particularly as a separate layer or layers between the phosphor layer and one or both of the electrodes. It is believed that the utilization of such high dielectric strength materials permits the storage of large amounts of electrical energy when the electric field is induced so that a large amount of energy can be discharged to or across the phosphor when the field collapses, thus resulting in the excitation of the phosphor crystals to produce a high light output. Theoretically then, the light output will be directly proportional to the dielectric constant of the materials disposed between the electrodes.

The object of the instant invention is to provide an improved electroluminescent device having a greater light output than heretofore attainable from either a theoretical or practical standpoint. This object, as well as other objects and advantages are accomplished according to the invention by dispersing a ferroelectric material of high dielectric constant between the electrode layers in an electroluminescent device. The ferroelectric material may be dispersed with the electroluminescent phosphor in a dielectric plastic binder or it may be dispersed separately in a binder and incorporated as an additional layer between the phosphor layer and one of the electrodes. It has been unexpectedly discovered that when such a ferroelectric material is incorporated in an electroluminescent device, the light output is greatly increased by a factor substantially greater than theretically explainable by consideration of the dielectric con ice stant of the material alone. Thus, for example, the light output of a panel incorporating a layer of barium titanate is greater than the light output of a panel substantially identical in structure except for the omission of the barium titanate layer even when both panels are excited by an alternating electrical field of substantially the same voltage and frequency.

The invention will be described in greater detail by reference to the drawings in which:

Fig. 1 is a cross-sectional, elevational view of one embodiment of an electroluminescent panel device wherein the ferroelectric and the electroluminescent materials are provided in separate layers;

Fig. 2 is a cross-sectional, elevational view of another embodiment of an electroluminescent panel device wherein the ferroelectric and the electroluminescent materials are mixed together and provided in one layer;

Fig. 3 is a graph illustrating the light output-voltage relationships of a pair of electroluminescent panel devices identical in structure except for the inclusion of a layer of barium titanate in one of the panel devices; and

Fig. 4 is a graph illustrating the light output-voltage relationships of another pair of electroluminescent panel devices identical in structure except for the inclusion of a layer of barium titanate in one of the panel devices.

Referring now to Fig. 1 an embodiment of an electroluminescent device is shown wherein the ferroelectric material is dispersed in a binder and applied as a separate layer distinct from the phosphor layer. The device comprises a base member 2 of transparent material such as glass or a plastic. If a plastic material is desired for the base, methyl methacrylate, for example, may be employed. A transparent conductive film 4 is applied over a surface of the transparent base. This transparent conductive film 4 may be formed by vapor-depositing gold, for example, onto the base 2. The transparent conductive film 4 is a very thin, electrically conductive layer, the rmistivity of which should be about ohms per square. A satisfactory method for producing such a transparent conductive film is to clamp a pair of electrodes to the surface of the transparent base during the vapor deposition of the metal whereby the resistivity of the film being deposited may be continuously observed with an ohmmeter until the desired resistance is obtained.

Preferably, a layer 6 of dielectric material is applied over the transparent conductive film 4. The dielectric layer 6 may be a plastic such as urea formaldehyde, for example. material dispersed in a binder is applied to form the electroluminescent layer 8 over the dielectric layer 6. The phosphor may be one selected from a large group of materials known in the art to produce luminescence when disposed in an alternating electric field. Zinc sulphide activated by manganese is a suitable example. The binder in which the phosphor powder is dispersed may also be commercially available urea formaldehyde, enamel or lacquer, for example. A layer 10 of ferroelectric material dispersed in a binder is then applied over the phosphor layer 8. This layer may be formed by mixing finely divided barium titanate, for example, with urea formaldehyde lacquer or enamel and a solvent therefor. Thereafter a conductive layer 12 which may or may not be transparent, as desired, is applied over the ferro-electric layer 10. This conductive layer 12 may be a transparent gold film which is vapor deposited in the manner heretofore described or it may be silver which is painted over the layer 10 by known techniques. The transparent conductive films which may be used in devices according to the invention are aluminum, gold, silver, tin, or tin oxide. These materials may be vapor-deposited to form the required transparent, electrically conductive layer.

Application of the dielectric layer 6 may be achieved.

Thereafter, an electroluminescent phosphor by thoroughly mixing 2 parts by weight of commercially available urea formaldehyde, enamel or lacquer with 1 part by weight of a suitable solvent such as toluene, for example. When solutions or mixtures with urea formaldehyde are mentioned herein, it should be understood that the parts by weight of commercially available urea formaldehyde are based upon a starting material which is actually 40% by weight of resin solids. This mixture may then be sprayed over the transparent conductive film 4 and thereafter the coated panel may be baked at about 85 C. for about 2 hours in order to drive oil any solvent and cure and harden the dielectric plastic layer. The electroluminescent layer '8 is applied over the dielectric layer 6 by spraying, for example, to a thickness of about 3 mils plus or minus 0.1. A suitable spray formulation comprises about 2 parts by weight of phosphor powder thoroughly mixed with about 5 parts by weight of commercially available urea formaldehyde enamel or lacquer, thinned to spraying consistency with tolueneisopropyl alcohol in equal parts. The phosphor may be manganese-activated zinc sulphide such as Du Pont phosphor (296-3253, sold by E. I. du Pont de Nemours and Company, Wilmington, Delaware. After application of the phosphor coating, the coated panel may then be baked again for 2 hours at about 85 C.

A suitable ferroelectric material may be barium titanate, for example. The titanate may be mixed with a binder such as urea formaldehyde enamel or lacquer in equal parts by weight, for example. It should be under stood that the percentage of the ferroelectric material in the binder should be as high as possible in order to achieve the utmost benefit therefrom. However, from a practical consideration, if the proportion of ferroelectric material to the binder is too great, then the binding action of the carrier medium is degraded and may be ineffective. In the case where urea formaldehyde is employed as the binder, up :to about two parts by weight of barium titanate with one part by weight of the cured binding agent (the resin solids in urea formaldehyde) could be dispersed therein without deleteriously afiecting the binding action.

Example I An electroluminescent panel such as shown in Fig. 1 was made up on a transparent conducting glass base memher about A thick by applying a layer 0.0018" thick of electroluminescent phosphor material on the base memher. The transparent conducting glass was of the type known in the art as Nesa glass. The electroluminescent layer was formed and applied by spraying a thorough mixture of 10 grams of greenemitting phosphor material and 38 grams of a dielectric medium comprising a solution made up as follows: 100 parts by weight of urea formaldehyde enamel, 25 parts by weight of toluene, and 25 parts by weight of isopropyl alcohol. The panel was then baked for about 2 hours at about 85 C. Thereafter a ferroelectric layer 0.0025" thick was applied over the phosphor layer by spraying a thorough mixture of grams of barium titanate and 38 grams of the urea formaldehyde-toluene-isopropyl alcohol solution just described. Thereafter, the coated panel was again baked for about 2 hours at about 85 C. An electrically conductive layer of silver paste was then applied over the ferroelectric layer by spraying.

Referring now to Fig. 3, the light output vs. voltage characteristic of this panel is represented by curve A. The panel was excited with an alternating electrical field of 400 cycles. Curve B represents the light output vs. voltage characteristic of a comparison panel made up substantially identically as the panel of curve A except that the electroluminescent layer was made up by spraying a solution of 20 grams of the same green-emitting phosphor material and 38 grams of the previously described urea formaldehyde-toluene-isopropyl alcohol solution to a'thickness of 0.0027 and a barium titanate layer was not included; that is, a layer of 0.0025" thick of the urea formaldehyde-toluene-isopropyl alcohol solution alone without barium titanate was'applied over the phosphor layer. Both panels were excited by the same alternating electrical field of 400 cycles. It will be observed that the light output of the panel of Example I is substantially greater than the li ht output of the comparison panel over voltages from 125 v. to 350 v., the average intensity in light being about 3.0 times as great for the panel of Example I. Since a greater amount of phosphor was included in the comparison panel than in the panel of Example I, it would be expected that a greater amount of light should have been produced. Such was not the case, however, as shown by the curves of Fig. 3.

Example II An electroluminescent panel such as shown in Fig. 1 was made up on a transparent conducting glass base member about thick by applying a layer 0.0026" thick or" electroluminescent phosphor material in a dielectric medium over the base member. The transparent conducting glass was of the type known in the art as Nesa" glass. The electroluminescent layer was formed and applied by spraying a thorough mixture of 10 grams of green-emitting phosphor material and 38 grams of a dielectric medium comprising a solution made up as follows: parts by weight of urea formaldehyde enamel, 25 parts by weight of toluene, and 25 parts by weight of isopropyl alcohol. The panel was then baked for about 2 hours at about 85 C. Thereafter, a ferroelectric layer 0.0020" thick was formed over the phosphor layer by spraying a thorough mixture of 20 grams of barium titanate and 38 grams of the urea formaldehyde-tolueneisopropyl alcohol solution just described. Thereafter, the coated panel was again baked for about 2 hours at about 85 C. and an electrically conductive layer of silver paste was applied over the ferroelectric layer by spraying.

Referring now to Fig. 4, Curve C represents the light output vs. voltage characteristic of this panel when excited with an alternating electrical field of 400 cycles. Curve D represents the light-output vs. voltage characteristic of a comparison panel made up substantially the same as the panel of curve C except that the electroluminescent layer was made up by spraying a solution of 20 grams of the same gram-emitting phosphor material and 38 grams of the previously described urea formaldehyde-toluene-isopropyl alcohol solution to a thickness of 0.0027 and no ferroelectric material was included in the coating of urea formaldehyde formed over the phosphor layer. Both panels were excited with a 400 cycle alternating electrical field. it will be noted that the light output of Example ll is substantially greater than the light output of the control panel over voltages from v. to 35 0 y., the average intensity in light being about 1.5 times as great. Again, since more phosphor material was included in the comparison panel than in the panel of Example II, it would be expected that a greater amount of light would be obtained therefrom. As in the case of Example I and its comparison panel, this was not the case, however.

Referring now to Fig. 2, another embodiment of an electroluminescent device is shown wherein the ferroelectric material is dispersed in a binder with the electroluminescent material. The device corresponds in general with the device of Fig. 1 except that there is no separate and distinct layer of ferroelectric material, it being incorporated into the layer of electroluminescent material. As shown, the base member 2 is provided with a transparent conductive fiLn 5 and a la 'er 6 of dielectric medium applied thereto as described in connection with the device of Fig. l. A layer is applied over the dielectric layer 6 and comprises a mixture of barium titanate, for example, and a manganese-activated zinc sulphide phosphor. The barium titanate and the phosphor material are thoroughly mixed and dispersed in a binder such as urea formaldehyde enamel or lacquer, for example, and sprayed over the layer 6. Thereafter, a conducti-ve layer 12 which may again be silver paste is applied over the layer 14 of ferroelectric and phosphor material.

Example III An electroluminescent panel such as shown in Fig. 2 was made up on a methyl methacrylate base member about thick by applying a layer 0.0018 thick of electroluminescent phosphor material and ferroelectric material in a dielectric medium over a transparent gold film of about 100 ohms resistivity disposed on the base member. The electroluminescent-ferroelectric layer was formed and applied by spraying a thorough mixture of 10 grams of yellow-emitting phosphor material and 5 grams of barium titanate with 38 grams of a dielectric medium comprising a solution made up as follows: 100 parts by weight of urea formaldehyde enamel, 25 parts by weight of toluene, and 25 parts by weight of isopropyl alcohol. The panel was then baked for about 2 hours at about 85 C. A layer 0.0010 thick of dielectric material was then applied over the ferroelectric electroluminescent layer by spraying the urea formaldehydetoluene-isopropyl alcohol solution just described thereover. The panel was then baked for about 2 hours at about 85 C. and thereafter an electrically conductive layer of silver paste was applied over the dielectric layer.

In comparison with an electroluminescent panel made up substantially identically with the panel of Example III, except for the omission of barium titanate in the phosphor layer, the light output of the panel of Example III was greater than the light output of the comparison panel.

As used herein and in the appended claims, and as applied to materials or layers, the words electroluminescent and phosphor are intended to mean a material or layer which will luminesce when disposed in an alternating electrical field. The Word dielectric is intended to describe a material which, in addition to being electrically insulating, has the property that the energy required to establish an electric field thereacross is recoverable in whole or in part as electric energy.

What is claimed is:

1. An electroluminescent device comprising a layer of electroluminescent material mixed with barium titanate disposed between a pair of electrically independent electrode layers.

2. An electroluminescent device comprising a layer of a mixture of electroluminescent material and barium titanate dispersed and suspended in a dielectric medium, said layer being disposed between a pair of electrically independent electrode layers.

3. An electroluminescent device comprising a pair of electrically conductive layers separated by a first layer of electroluminescent material dispersed and suspended in a dielectric medium and a second layer of barium titanate dispersed and suspended in a dielectric medium, said electrically conductive layers being electrically independent.

4. An electroluminescent device comprising a base member, a transparent layer of electrically conductive material disposed on said base member, a layer of electroluminescent material disposed on said transparent layer, a layer of barium titanate disposed on said layer of electroluminescent material, and a layer of electrically conductive material disposed on said layer of barium titanate, said layers of electrically conductive material being electrically independent.

5. An electroluminescent device comprising a base member, a transparent layer of electrically conductive material disposed on said base member, a layer of a mixture of electroluminescent material and barium titanate disposed on said transparent layer, and a layer of electrically conductive material disposed on said layer of said mixture, said layers of electrically conductive material being electrically independent.

6. An electroluminescent device comprising a base member, a transparent layer of electrically conductive material disposed on said base member, a layer of electroluminescent material dispersed in a dielectric medium and disposed on said transparent layer, a layer of barium titanate dispersed in a dielectric medium and disposed on said electroluminescent layer, and a layer of electrically conductive material disposed on said layer of barium titanate, said layers of electrically conductive material being electrically independent.

7. An electroluminescent device comprising a base member, a transparent layer of electrically conductive material disposed on said base member, a layer of a mixture of electroluminescent material and barium titanate dispersed in a dielectric medium and disposed on said transparent layer, and a layer of electrically conductive material disposed on said layer of said mixture, said layers of electrically conductive material being electrically independent.

8. An electroluminescent device comprising a base member, a transparent layer of electrically conductive material disposed on said base member, a layer of dielectric material disposed on said transparent layer, a layer of electroluminescent material disposed on said layer of dielectric material, a layer of barium titanate disposed on said electroluminescent layer, and a layer of electrically conductive material disposed on said layer of barium titanate, said layers of electrically conductive material being electrically independent.

9. The invention according to claim 8 wherein said electroluminescent material and said barium titanate are dispersed in a dielectric medium.

10. An electroluminescent device comprising a pair of electrically independent electrode layers with a layer of electroluminescent material and a layer of barium titanate disposed therebetween.

References Cited in the file of this patent UNITED STATES PATENTS 2,624,857 Mager Jam 6, 1953 2,721,808 Roberts et al. Oct. 25, 1955 2,824,992 Bouchard et a1. Feb. 25, 1958 

4. AN ELETROLUMINESCENT DEVICE COMPRISING A BASE MEM,BER, A TRANSPARENT LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL DISPOSED ON SAID BASE MEMBER, A LAYER OF ELECTROLUMINESCENT MATERIAL DISPOSED ON SAID TRANSPARENT LAYER, A LAYER OF BARIUM TITANATE DISPOSED ON SAID LAYER OF ELECTROLUMINESCENT MATERIAL, AND A LAYER OF ELECTRI- 